wifi: mt76: mt7996: fill txd by host driver

The hardware SDO has issue to fill txd for the moment, so fallback to
driver filling method.

Fixes: 98686cd2 (wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices)
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>